SUMMARY The growing prevalence of obesity and associated type II diabetes is a major health concern, particularly among children. Recent evidence has shown that obesity might be a consequence of alterations in the developmental processes of systems involved in energy balance regulation, including the hypothalamic melanocortin system. This neural system, which includes neurons in the arcuate nucleus that produce pro- opiomelanocortin (POMC), acts a major negative regulator of energy balance. The complex patterns of neuronal wiring in the adult hypothalamus depend on a series of developmental events that establish a framework on which functional circuits can be built. We previously reported that POMC projections develop during the first 3 weeks of postnatal life. Growing POMC axons must then choose a path to follow and must decide the direction to go on this path to innervate the proper nucleus (e.g., the paraventricular nucleus, PVH). The rate and direction of axon growth are defined by diffusible axon guidance cues. Neuropilin (Nrp) is a neuronal cell surface protein that has been shown to function as a receptor for class 3 semaphorins (Sema3s), which are secreted proteins that influence axon guidance. Recent exome sequencing experiments have identified novel variants in the Sema3 and Nrp families that are associated with severe obesity in humans. The overall hypothesis of this proposal is that neuropilin-semaphorin signaling mediate the development of hypothalamic POMC circuits involved in energy homeostasis. We also hypothesize that Sema3 and Nrp mutations impair their chemoattractive properties and that their overexpression in specific components of the melanocortin system disrupts development of POMC axonal projections and causes metabolic perturbations. The following specific aims will be addressed. Specific Aim 1. We will use in vitro co-culture assays to test the effect of human Sema3 mutations on POMC axon growth. Specific Aim 2. We will generate mouse models in which Nrp1 and/or Nrp2 are deleted in postnatal POMC neurons to evaluate the importance of Nrp1/2 signaling in the formation of POMC circuits. We will also evaluate how loss of Nrp1/Nrp2 signaling in POMC neurons impacts energy balance regulation. We will then explore the neurodevelopmental and metabolic consequences of the overexpression of human Nrp variants in POMC neurons. Specific Aim 3. In this specific aim, we will focus on Sema3C. We will generate mouse models in which Sema3C is deleted in the PVH to explore the importance of Sema3C expression in the development of POMC?PVH circuits. We will also examine the physiological role of PVH Sema3C expression in energy and glucose balance. Finally, we will investigate the neurodevelopmental and metabolic consequences of the overexpression of a human Sema3C variant in the PVH. Together, this work will provide new insight into the molecular mechanisms underlying the development of key neural systems involved in metabolic regulation and may have translational impact on human health.